1. Field of the Invention
The present invention relates to a method for preventing polymerization of acrylic acid during distillation thereof, and more particularly to a method for preventing polymerization of acrylic acid, during azeotropic dehydration distillation of an acrylic acid aqueous solution in an acrylic acid production process, due to by-products present in the acrylic acid aqueous solution (feed liquid) and for preventing deposition of the by-products in the distillation column.
2. Description of the Prior Art
An enormously large amount of acrylic acid has been produced in large-scale manufacturing plants. A typical process of manufacturing the same is described below. First, propylene and/or acrolein (hereinafter, referred to as “propylene and the like”) are brought into contact in a gas phase with a molecular oxygen-containing gas, and catalytically oxidized to give a reaction gas. The reaction gas contains, together with the desired product acrylic acid, various compounds including unreacted acrolein and by-products such as formaldehyde, glyoxal, furfural, benzaldehyde, formic acid, acetic acid, and maleic acid. Subsequently, the reaction gas was brought into contact with and absorbed in an absorption liquid, such as water, giving an acrylic acid aqueous solution, and the impurities such as the by-products and water contained in the acrylic acid aqueous solution were removed by means of distillation or the like to produce high purity acrylic acid. As there is not a sufficiently large difference in relative volatility between acrylic acid and water or between acrylic acid and acetic acid, it is difficult to have high-purity acrylic acid in a simple distillation process. Accordingly, an azeotropic distillation process, wherein a mixture of an acrylic acid aqueous solution and an azeotropic solvent is distilled together, is typically used as the distillation process. In addition, a temperature of the distillation needs to be increased for removing the impurities, which raises a problem that acrylic acid itself tends to polymerize during the distillation. Therefore, a variety of polymerization inhibitors are introduced into the distillation column for the purpose of preventing the polymerization of acrylic acid therein and ensuring a consistent operation of the distillation for an extended period of time.
As such processes are known: a process for preventing the polymerization of acrylic acid by introducing into the distillation column N-nitrosophenylhydroxyamine or the salt thereof along with a copper salt compound (see Japanese Unexamined Patent Publication No. 9-95465); and a process for preventing the polymerization of acrylic acid by introducing into the distillation column a three-component polymerization inhibitor consisting of an N-oxyl compound, a phenol compound, and a phenothiazine compound along with molecular oxygen (see Japanese Unexamined Patent Publication No. 6-345681).
A variety of other polymerization inhibitors along with those described above have been proposed as the means for preventing the polymerization of acrylic acid, but each of the polymerization inhibitors, although highly effective for prevention of polymerization of acrylic acid during storage, is not so dominant for preventing the polymerization of acrylic acid during distillation, especially during azeotropic dehydration distillation (herein after may be called as azeotropic dehydration). In addition, as described later, the present inventors have found that these polymerization inhibitors are not effective for prevention of deposition of by-products contained in the acrylic acid aqueous solution. For an commercial-scale production of acrylic acid, which requires operation over quite a long period of time, a polymerization inhibitor that retains an excellent potential for preventing the polymerization of acrylic acid for an extended period of time is desired, but there is no such inhibitors available yet that could satisfy the need.
An object of the present invention is to provide a method for effectively suppressing the polymerization of acrylic acid and the deposition of by-products during an azeotropic dehydration distillation in an acrylic acid manufacturing process and ensuring a consistent operation of the azeotropic dehydration distillation column over an extended period of time.